Remove deprecated `govuk-header__link--service-name` class

[36degrees]

What

Remove the govuk-header__link--service-name class from the header component.

Tell users to replace instances of govuk-header__link--service-name with govuk-header__service-name.

Why

The govuk-header__link--service-name was deprecated in #2617 when the new equivalent class name govuk-header__service-name was introduced.

The govuk-header__link--service-name class was a modifier for links, but the other changes in the PR meant that it was no longer always going to be used on a link, so we renamed it.

Who needs to work on this

Developer, tech writer

Who needs to review this

Developer, tech writer

Done when

• [ ] The govuk-header__link--service-name class has been removed
• [ ] Breaking change documented in release notes with a clear path for migration